Method of making enamelware



Patented Oct. 3, 1944' William J. Baldwin, Ingram, Pa.,

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a zn Homelaya, Incorporated, Pittsburgh, Pa", a corporation of Pennsylvania so Drawing. Original application April 12, 1939,

Divided and this application February 13, 1942, Serial No. 430,755 I Serial No. 267,523.

2 Claims.

rected to the production of an article of enamelware of unique surface configuration and decorative appearance, and it consists in a, method of procedure whereby a product of distinctive appearance is gained. This application is divided from an application filed by me April 12, 1939, Serial No. 267,523, now Patent No. 2,290,795.

In the prevailing practice of enamel-ware making, a suitable batch of material is smelted to glass, the glass is cooled, solidified, and crushed. The crushed glass, called frit, is worked with clay and water to a paste called slip, and the slip is applied to the metal surface, typically by dipping. The coating so spread upon the surface of the article is dried to hardness, and then the coated article is heated to a temperature at which the glass melts again, takes .up the clay as a modifying component, and forms more intimate union with the underlying metal. On cooling, the melted glass hardens to an integral and adherent coating. A second and a third coating may similarly be developed upon the first.

In Letters Patent of the United States, No. 2,043,559, granted June 9, 1936, on the application of Jacob E. Rosenberg, a refinement in the method of forming the initial coating upon enamel-ware articles is described that involves the addition to the batch of glass-forming materials of the antimonate of an alkali or of an alkaline-earth metal (typically sodium antimonate. I have been engaged in further investigation along lines laid down by the said Rosenberg patent.

I have found it convenient, instead of introducing sodium antimonate as such with the other materials that go to make up the batch, to introduce antimony trioxide (Sb40a) together with nitrate of soda (NaNOz), with the consequence that in the smelting operation sodium antimonate forms itself by spontaneous reaction.

My present invention proceeds from the discovery that, in a batch that includes a fluoride, if the antimony trioxide addition be in excess quantity, so that there remains in the frit and in the coating that is' eventually melted upon the metal surface a quantity of antimony trioxide (the figures will be given below) that has not by oxidation been brought to the pentavalent form, and if the ultimate firing (that is to say, the melting of the coating of dried and hardened slip upon the metal) be carried out in an oxidizing atmosphere, an unexpected result will ensue, that gives to the finished article a unique and highly decorative feature of appearance. skin apparently forms upon the air interface of the molten layer of coating, and in cooling and solidification of the layer the underlying body of glass (it would seem) contracts slightly, with the-effect of causing the'skin to wrinkle and the wrinkles to organize themselves into groups radiating from centres that seem to be determined by chance. The minutely waved surface, in which order and chance are combined, gives to the finished article a surface configuration that, under the light of day, modifies'the monotony, and imposes upon the otherwise plain and colored surface a pattern that closely resem'bles the surface pattern of galvanized iron. This surface pattern I shall, for the sake of brevity, characterize the crinkle finish.

Nitrate of soda (NaNQa) 3.2

' In the smelting, while there is a small loss in total weight, there is no loss in the weights of the cryolite and the antimony trioxide constituents; and it will be found that in the frit these two constituents will continue cryolite, substantially 10.5 and antimony trioxide, substantially 8.5%the antimony trioxide reacted, as already explained,. to sodium antimonate.

In normal use of the formula, when the batch is compounded and all the ingredients are smelted together, the resulting enamel-ware presents a plain and uniform appearance of surface.

The experiment that led to the discovery consisted in compounding the batch according to the formula, including the antimony trioxide, but omitting the nitrate of soda, and in adding the nitrate of soda, only after the smelting of the other ingredients had been accomplished. Upon the procedure so modified the unique and wholly unexpected appearance of the product, characterized above, ensued. Recognizing its value, I proceeded to investigate and determine the limiting conditions of its occurrence.

If, in place of antimony trioxide and nitrate oi soda, (sodium antimonate (NaSbOa) be included ini or added to the batch (the antimony assumingin the melt its pentavalent condition- SbzOs) the crinkle finish will not appear.

If the formula be modified, either by increase in the antimony trioxide content or reduction in the nitrate of soda content, until the ratio of antimony trioxide to nitrate appreciably exceeds 4:1 (procedure being normal and all the constituents being smelted together), the crinkle finish will appear in the product. Thus, if the antimony trloxide content be held at 8.5 parts and the nitrate of soda content be gradually reduced in successive runs, it will be found that the crinkle finish will appear when the nitrate content has fallen below 2 parts, and will be yet more pronounced at 1 part, 0.50 part, etc. And, again, if the nitrate content be held at 2.5 parts and the antimony trioxide content be gradually increased, the crinkle finish will appear when the trioxide content reaches 11 parts, and will be yet 4 more pronounced as the trioxide' content rises ceed the nitrate of soda'content in a ratio of substantially 8.5:2.

In the cryolite content of the batch is reduced from the typical 10.5 parts to as little as 6 parts, the crinkle finish appears in less pronounced degree. If both the antimony trioxide content and the cryolite content be reducedthe antimony trioxide content to 6 and the cryolite content to 8 (the sodium nitrate being absent)the crinkle finish tends to disappear.

If coloring material of a reducing nature be added, the crinkle finish is correspondingly lost. If the article be fired in a non-oxidizing atmosphere, the crinkle finish will not develop. All of which indicates the formation in the firing of the article, perhaps by oxidation, of a skin at the air interface, and that it is the skin so formed that, as the body of the coating shrinks in cooling, is thrown into the minute wrinkles that characterize the crinkle finish. My observations lead me to believe that the crinkle finish develops at the very beginning of the cooling of the article, and that sudden cooling through a wide temperature range is not necessary to achieve the result.

These investigations of mine lead to the can clusion that, whereas the results that Rosenberg has in view in letters Patent No. 2,043,559 are gained by the reaction of antimonate, present in the glass when the coating or irit is fired upon the metal article, and whereas the presence of antimonate may be insured by including antimony trioxide in the batch in association with an oxidizing agent, the crinkle finish that I have discovered is achieved when and only when the batch includes a fluoride and when antimony trioxide in excess is present in the batch and persists in trivalent state in the frit.

I claim as my invention:

1. The method herein described of producing a metal article having a vitrified coating with a crinkle finish which comprises applying to the metal article a vitrifiable coating compound that contains a irit of usual composition including cryolite and which i'rit also includes unreacted antimony trioxide, firing the article so coated in an oxidizing atmosphere to vitriiy the coating and under conditions which preserve in the finished coating some unreacted antimony trioxide, and cooling the ware after firing.

2. The method herein described of producing a metal article having a vitrified coating with a crinkled finish which comprises applying to the metal article a vitrifiable coating compound that contains a frit of usual composition but wherein the cryolite is present in substantially 10.5 parts in a hundred, fiuorspar substantially 4.3 parts in a hundred, the trit also having present therein antimony trioxide to the amount of at least 6 parts in a hundred, firing the coated article in an oxidizing atmosphere to vitrity the coating under conditions which preserve in the finished coating some unreacted antimony trioxide, and cooling the ware after firing.

WILLIAM J. BALDWIN. 

